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ESD Brushes & Static Electricity

The need to solve static generation and dissipative problems has grown, not because of the increase in the computer chip use, but increased density of the chips. As industry puts more and more on a single chip, the layers inside the chips are getting smaller and smaller, making them more susceptible to the effects of ESD and EOS. When static charge of less than 10 volts will destroy a class 1 ESD sensitive device and it is possible to create a static charge of over 35000 volts walking across a carpet on a dry day, the problem is immediate and severe. Gordon Brush is solving this problem with ESD Brushes.

Static charge is generated between materials through friction, pressure, or separation of two materials, one of which is usually non-conductive. This process is called the Triboelectric Effect [tribo means rubbing]. The actual level of charge is measured in coulombs. Commonly, however, the electrostatic potential on an object is expressed as a voltage.

In addition to the material composition, applied forces and separation rate, relative humidity is an important factor in the generation of static charges.

When humidity is low, higher static charges are generated more easily. Static becomes more noticeable in the winter months, in dry climates, and in air-conditioned environments. Increasing humidity to 60% limits static build-up as surface moisture on materials makes a good conductor. Unfortunately, 60% relative humidity is extremely uncomfortable, can cause equipment problems, and introduce contaminants.

There are two sources of static damage to electronic devices: EOS [Electrostatic Overstress] and ESD [ElectroStatic Discharge].

EOS occurs when an electronic device is exposed to a strong static electric field generated by a static charge. Internal electronic components stressed beyond their designed tolerances may fail immediately or have reduced service lives.

ESD occurs when two objects of different electrostatic potentials are brought close enough together to allow a charge transfer. ESD is the sudden discharge of this electrostatic potential from one body to another. If the discharge current exceeds designed tolerances, damage may cause immediate failure or result in reduced service life.

ESD Brush Material Definitions & Properties

Materials for static control of brushes are placed in one of three classes: Conductive, Dissapative, and Insulative. Additionally, there is a commonly accepted category referred to as Antistatic.

Conductive Brush Materials allow charges to move freely across their surfaces or through their volume. Charge placed in one spot on a conductive object will flow around the object so that all parts of the object are equally charged. If a charged conductor is grounded, charges will recombine [move to ground] until the object has no charge. Conductive materials have a low resistance to current flow. One common misconception is that conductive materials do not generate charges. This is because the dissipation of static charges from grounded conductive material tends to be complete and rapid. Ungrounded conductors can generate and hold static charges.

Dissipative Brush Materials allow charges to move more slowly across their volume. Charge placed in one spot on a dissipative object will flow around the object so that all parts of the object share the same charge. If a charged dissipative material is grounded, charges will recombine at a slower rate until the object has no charge. Dissipative materials have a resistance to current flow that is greater than conductors, but less than insulators.

 
Insulative Brush Materials do not allow charges to move across their surfaces or through their volume. Charge placed in one spot on an insulative object will stay in that location. If a charged insulator is grounded, charges will not move to ground. Grounding is not an effective method of neutralizing insulators. Insulators can have both negatively and positively charged areas on the same object. Because insulators do not allow charge movement, they can accumulate massive amounts of charge. Static fields on insulators are not necessarily permanent; they will eventually be neutralized by gradual recombination with free ions.

Antistatic Brush Materials as defined by the EIA and the ESD Association, refers to the property of a material that inhibits triboelectric charging. A material that inhibits the generation of static charges [generally less than 200 volts] from triboelectric generation is classified as antistatic. These materials would be arranged in the triboelectric series near cotton, an electrically neutral material. Materials ordered in either side of cotton generate some static charge, but in very small amounts. Note: a material's anti-static characteristic does not necessarily correlate with its resistivity. An antistatic material can be conductive, dissipative, or even insulative. Only antistatic or dissipative antistatic materials should be used in ESD safe areas. Only contact with a conductive material can damage a charged device.


Electrical Conductivity of Metals
Copper Aluminum Alloy
(Soft Annealed)
Brass Nickel Silver Phosphor Bronze Steel
100 45 - 50 28 5.3 36 3 - 15

 

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